netdev - Re: [PATCH] vhost: get 2% performance improved by reducing spin_lock race in vhost_work

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Date:	Mon, 20 May 2013 04:22:11 +0000
From:	Qinchuanyu <qinchuanyu@...wei.com>
To:	Jason Wang <jasowang@...hat.com>
CC:	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"mst@...hat.com" <mst@...hat.com>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"(kvm@...r.kernel.org)" <kvm@...r.kernel.org>,
	"(netdev@...r.kernel.org)" <netdev@...r.kernel.org>,
	Heguansen <heguansen@...wei.com>
Subject: Re: [PATCH] vhost: get 2% performance improved by reducing
 spin_lock race in vhost_work_queue

The patch below is base on 
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/drivers/vhost/vhost.c?id=refs/tags/next-20130517

Signed-off-by: Chuanyu Qin <qinchuanyu@...wei.com>
--- a/drivers/vhost/vhost.c     2013-05-20 11:47:05.000000000 +0800
+++ b/drivers/vhost/vhost.c     2013-05-20 11:48:24.000000000 +0800
@@ -154,9 +154,10 @@
        if (list_empty(&work->node)) {
                list_add_tail(&work->node, &dev->work_list);
                work->queue_seq++;
+               spin_unlock_irqrestore(&dev->work_lock, flags);
                wake_up_process(dev->worker);
-       }
-       spin_unlock_irqrestore(&dev->work_lock, flags);
+       } else
+               spin_unlock_irqrestore(&dev->work_lock, flags);
 }
 
 void vhost_poll_queue(struct vhost_poll *poll)

I did the test by using iperf in 10G environment, the test num as below:
                 orignal                   modified
thread_num  tp(Gbps)   vhost(%)  |  tp(Gbps)     vhost(%)
1           9.59         28.82   |      9.59        27.49
8            9.61        32.92   |      9.62        26.77
64            9.58        46.48  |     9.55        38.99
256            9.6        63.7   |      9.6         52.59

The cost of vhost reduced while the throughput is almost unchanged.

On 05/20/2013 11:06 AM, Qinchuanyu wrote:
> Right now the wake_up_process func is included in spin_lock/unlock, but it could be done outside the spin_lock.
> I have test it with kernel 3.0.27 and guest suse11-sp2, it provide 2%-3% net performance improved.
>
> Signed-off-by: Chuanyu Qin <qinchuanyu@...wei.com>

Make sense to me but need generate a patch against net-next.git or
vhost.git in git.kernel.org.

Btw. How did you test this? Care to share the perf numbers?

Thanks
> mu
> --- a/drivers/vhost/vhost.c     2013-05-20 10:36:30.000000000 +0800
> +++ b/drivers/vhost/vhost.c     2013-05-20 10:36:54.000000000 +0800
> @@ -144,9 +144,10 @@
>         if (list_empty(&work->node)) {
>                 list_add_tail(&work->node, &dev->work_list);
>                 work->queue_seq++;
> +               spin_unlock_irqrestore(&dev->work_lock, flags);
>                 wake_up_process(dev->worker);
> -       }
> -       spin_unlock_irqrestore(&dev->work_lock, flags);
> +       } else
> +               spin_unlock_irqrestore(&dev->work_lock, flags);
>  }
>  
>  void vhost_poll_queue(struct vhost_poll *poll)
> --
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